1. Field of the Invention
This invention relates to defect detection using scattered light and more specifically to automatically detecting macro defects on surfaces such as semiconductor wafers, liquid crystal display panels, disk drive platters and the like using scattered light.
2. Description of the Related Art
A well known problem in the semiconductor industry, and other industries where surface defects are a problem, is the need to inspect for macro defects on the active surface e.g. the surface on which circuit elements, electro-magnetic coatings or optical coatings are formed. Typically, these inspections take place numerous times during the fabrication process and can be as often as several times for each fabricated layer. Macro defects can result from scratches and/or particles and typically can range in size from microns (.mu.m) to a millimeter (mm) or more. Such defects result in a portion of the device being fabricated to function improperly, or not to function at all. Hence, it is important that inspections that can detect macro defects be performed periodically during the fabrication process. In addition, when defects are found it is important to pinpoint the location of each defect to allow that portion of the surface to be indicated as not usable, where possible.
Some prior art relies on manual inspections. Light, typically white light, is directed toward the surface of the substrate from an oblique angle. The presence of a macro defect can cause scattering of the light. An example of such a manual system that is used for visual front and back macro-inspections of semiconductor wafers is the Brightlight 200 manufactured by Irvine Optical Corporation of Burbank, Calif. However, a problem with such prior art systems is that light scattering can result from previous processing, and not indicate a defect. For example, when a semiconductor wafer surface is inspected, the patterns of previous layers created during the fabrication process cause light diffraction; thus an operator is required to distinguish these previous layer signals from a defect. Hence, the ability of that operator to distinguish a macro defect can change as a function of prior layer processing. Thus, manual inspections are subjective and prone to variable results due to fatigue or other factors. In addition, manual inspections are time consuming and often only a portion of the samples are inspected.
Other prior art defect detection systems use an automated image processing and comparison method known as "Golden Template Comparison" (GTC). GTC is described by William M. Silver in Golden Template Comparison, Sensors, October 1990, pp. 20-6, which is incorporated by reference herein. Additionally, GTC is available, as a combination of hardware and software, with the Cognex 5600 Vision Processor from Cognex Corp. of Needham, Mass. However, while GTC is a viable method for defect detection in some environments, several problems emerge when GTC is used for inspection of surfaces having complex patterns applied by previous processing using scattered light. For example, GTC can often indicate false defects when sample images contain only minor intensity or color variations. In addition, GTC can also return false defects when star burst patterns caused by diffracted light are present.
Therefore, it would be advantageous to have a method of macro defect detection that does not require a manual inspection or if automated, generate false defect signals. It would also be advantageous to have a method that uses objective criteria for defect detection. Such criteria should not be compromised by false defect detection problems. In addition, it would be advantageous to have a macro defect detection method that can inspect surfaces at a uniformly high throughput rate thus allowing 100% of the samples to be inspected. Finally it would be advantageous to have a macro defect detection system that can be used in a variety of fabrication environments such as semiconductor wafer fabrication, liquid crystal display element fabrication and the like as both a stand alone inspection station or as a portion of some other processing system.
This application is related to previously filed U.S. patent application Ser. No. 08/644,649 filed on May 7, 1996 now U.S. Pat. No. 5,777,792 incorporated herein by reference, and assigned to the assignee of the present invention.